Here at Walden Heights, we are having trouble with plum trees. The plums in our orchard are relatively young and generally healthy and vigorous. They flower beautifully and are surrounded by pollinators, yet they bear no fruit. As we pruned this winter, we noticed some bacterial canker on the branches, characterized by gumosis and orange mold-looking spots. In the spring, the flowering stems were hit with blossom blast immediately after flowering. The blossom blast caused rapid brown dieback, often overnight. These symptoms were showing up on plums, cherry plums, and Hanson’s bush cherries, all closely related Prunus species.
After some research, we learned that these symptoms stem from the same bacterium- Pseudomonas syringae. This bacterium has over 50 pathovars, affecting a wide variety of plants including tomatoes, maples, wheat, mangoes, kiwi and more. Prunus stone fruits are affected by the P. syringae pathovars syringae (causing blossom blast) and mons-prunorum (causing bacterial canker).
P. syringae is present in the air, water, and on plant surfaces all around us. This bacterium causes water to freeze at high temperatures and is responsible for a majority of frost damage in plants. It also plays a big part in the Earth’s hydrological cycle. When airborne it serves as a cloud condensation nuclei and ends up in raindrops and hailstones.
Here in the orchard it’s a big nuisance. The pathogen lands on healthy stems, overwintering on healthy dormant buds. Because it causes ice to form at high temperatures it creates severe frost damage. In the spring when the damaged tissue thaws out, P. syringae colonizes that tissue. It colonizes especially quickly in cool, wet conditions. The pathogen will kill flowering stems immediately after flowering (blossom blast), move into the wood and create cankers. The pathogen will multiply and overwinter in these cankers, spreading further in the tree the following season.
There are no good treatments for this bacterium and prevention is the best strategy. Remove diseased stems, leaves, and cankers immediately to prevent it from spreading. Healthy trees can fight off the bacterium more successfully so invest in the health of your tree from the start. Make sure that the soil around the tree is full of microbes and that the tree is not stressed due to lack of nutrients or water. However, be sure not to over-fertilize. Too much fertilizer will create an abundance of succulent shoots, which are favorite overwintering spots for for the P. syringae bacterium. Prune the branches to ensure lots of air flow and quick drying of the tree’s surfaces, which reduces opportunities for the bacterium to colonize.
Foliar sprays that inoculate the shoots and leaves with probiotics are another way to invest in the health of the tree and help it fight off the bacterium. We are going to experiment with these sprays to improve the health and production of our plums.
Several extension agencies recommend spraying with copper, a common treatment for fungal and bacterial diseases. While there are some organic copper sprays available, they are harmful to beneficial populations and it is expensive to maintain full coverage of the tree. Full conventional orchards go so far as to spray with methyl bromide, but we do NOT recommend this treatment as it is highly toxic and not getting to the root of the problem.
Experiment with different varieties of plums! Plums tend to be very capricious- flourishing in one micro-climate and suffering in another- even when they are fairly close by. Talk with local growers and gardeners to find out what varieties do well for them try them on different areas of your property.
There’s nothing like a the deep flavor and juicy goodness of a plum in the summer, so we will continue to experiment and learn so that we can grow them successfully in our orchard. Please let us know if you are growing plums and what varieties are doing well for you!
Bacterial canker. 2009. http://www.which.co.uk/documents/pdf/bacterial-canker-151467.pdf (June 11, 2012)
Eastwell, Kenneth C. et. al. 2005. Field Guide to Sweet Cherry Diseases in Washington. Washington State University Extension. http://cru.cahe.wsu.edu/CEPublications/eb1323e/eb1323e.pdf (June 11, 2012)
Kennelly, Megan et. al. 2007. Psudomonas syringae Diseases of Fruit Trees. Plant Disease. http://www.agroquimicosgaspar.com.ar/wp-content/uploads/2011/12/Pseudomonas-en-frutales.pdf (May 31, 2012)
UC Management Guidelines for Bacterial Canker on Plum, 2009. University of California Davis. http://www.ipm.ucdavis.edu/PMG/r611100111.html (June 11, 2012)
prepared by Sarah Claassen (staffmember, apprentice, and friend) 2012